1. Technical Field
This application relates to managing accesses to storage objects.
2. Description of Related Art
Computer systems may include different resources used by one or more host processors. Resources and host processors in a computer system may be interconnected by one or more communication connections. These resources may include, for example, data storage devices such as those included in the data storage systems manufactured by EMC Corporation. These data storage systems may be coupled to one or more servers or host processors and provide storage services to each host processor. Multiple data storage systems from one or more different vendors may be connected and may provide common data storage for one or more host processors in a computer system.
A traditional storage array (herein also referred to as a “data storage system”, “disk storage array”, “disk array”, or simply “array”) is a collection of hard disk drives operating together logically as a unified storage device. Storage arrays are designed to store large quantities of data. Storage arrays typically include one or more storage array processors (SPs), for handling requests for allocation and input/output (I/O) requests. An SP is the controller for and primary interface to the storage array.
A storage array may be thought of as a system for managing a large amount of a resource, i.e., a large number of disk drives (also referred to as “disks” or “drives”). Management of the resource may include allocation of a portion of the resource in response to allocation requests. In the storage array example, portions of the storage array may be allocated to, i.e., exclusively used by, entities that request such allocation.
Data storage systems, such as disk drives, disk storage arrays, network storage devices, storage area networks, and the like, are called upon to store and manage a significant amount of data (e.g., gigabytes, terabytes, petabytes, etc.) that is written and read by many users. Storage arrays are typically used to provide storage space for a plurality of computer file systems, databases, applications, and the like. For this and other reasons, it is common for physical storage arrays to be logically partitioned into chunks of storage space, called logical units, or LUs. This allows a unified storage array to appear as a collection of separate file systems, network drives, and/or volumes.
A host processor may perform a variety of data processing tasks and operations using the data storage system. For example, a host processor may perform basic system I/O operations in connection with data requests, such as data read and write operations.
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units, disk drives, and disk interface units. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units (“logical volumes” or “LUNs”). The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data in the device. In order to facilitate sharing of the data on the device, additional software on the data storage systems may also be used.
Performance of a storage system can be characterized by the system's total capacity, response time, throughput, and/or various other metrics. The capacity of a storage system is the maximum total amount of data that can be stored on the system. The response time of a storage system is the amount of time required to read data from or write data to the storage system. The throughput of a storage system is a measure of the amount of data that can be transferred into or out of (i.e., written to or read from) the storage system over a given period of time.
The administrator of a storage array can desire to optimize the storage system in a manner that maximizes performance or balances cost vs. performance. In general, performance of a storage system can be constrained by both physical and temporal constraints. Examples of physical constraints include bus occupancy and availability, excessive disk arm movement, and uneven distribution of load across disks or across RAID groups (which are described below). Examples of temporal constraints include bus bandwidth, bus speed, spindle rotational speed, serial versus parallel access to multiple read/write heads, and the size of data transfer buffers.
One factor that can limit the performance of a storage system is the performance of each individual storage device. For example, the read access time of a storage system including hard disk drives is constrained by the access time of the disk drive from which the data is being read. Read access time can be affected by physical characteristics of the disk drive, such as the number of revolutions per minute of the spindle: the faster the spin, the less time it takes for the sector being read to come around to the read/write head.
Furthermore, even if a disk-based storage system uses the fastest disks available, the performance of the storage system can be limited by the number of those disks that can be accessed at a time. In other words, performance of a storage system, whether it is an array of disks, tapes, flash drives, or other storage devices, can also be limited by system constraints, such the number of data transfer buses available in the system and the density of traffic on each bus.
In the industry there have become defined several levels of RAID systems that may be used by groups of disks referred to as RAID groups. RAID (Redundant Array of Independent or Inexpensive Disks) parity schemes may be utilized to provide error detection during the transfer and retrieval of data across a storage system. The first level, RAID-0, combines two or more drives to create a larger virtual disk. In a dual drive RAID-0 system one disk contains the low numbered sectors or blocks and the other disk contains the high numbered sectors or blocks, forming one complete storage space. RAID-0 systems generally interleave the sectors of the virtual disk across the component drives, thereby improving the bandwidth of the combined virtual disk. Interleaving the data in that fashion is referred to as striping. RAID-0 systems provide no redundancy of data, so if a drive fails or data becomes corrupted, no recovery is possible short of backups made prior to the failure.
RAID-1 systems include one or more disks that provide redundancy of the virtual disk. One disk is required to contain the data of the virtual disk, as if it were the only disk of the array. One or more additional disks contain the same data as the first disk, providing a “minor” of the data of the virtual disk. A RAID-1 system will contain at least two disks, the virtual disk being the size of the smallest of the component disks. A disadvantage of RAID-1 systems is that a write operation must be performed for each minor disk, reducing the bandwidth of the overall array. In a dual drive RAID-1 system, the first disk and the second disk contain the same sectors or blocks, each disk holding exactly the same data.
RAID-2 systems provide for error correction through hamming codes. The component drives each contain a particular bit of a word, or an error correction bit of that word. RAID-2 systems automatically and transparently detect and correct single-bit defects, or single drive failures, while the array is running. Although RAID-2 systems improve the reliability of the array over other RAID types, they are less popular than some other systems due to the expense of the additional drives, and redundant onboard hardware error correction.
RAID-4 systems are similar to RAID-0 systems, in that data is striped over multiple drives. For example, the storage spaces of two disks are added together in interleaved fashion, while a third disk contains the parity of the first two disks. RAID-4 systems are unique in that they include an additional disk containing parity. For each byte of data at the same position on the striped drives, parity is computed over the bytes of all the drives and stored to the parity disk. The XOR operation is used to compute parity, providing a fast and symmetric operation that can regenerate the data of a single drive, given that the data of the remaining drives remains intact.
RAID-3 systems are essentially RAID-4 systems with the data striped at byte boundaries, and for that reason RAID-3 systems are generally slower than RAID-4 systems in most applications. RAID-4 and RAID-3 systems therefore are useful to provide virtual disks with redundancy, and additionally to provide large virtual drives, both with only one additional disk drive for the parity information. They have the disadvantage that the data throughput is limited by the throughput of the drive containing the parity information, which must be accessed for every read and write operation to the array.
RAID-5 systems are similar to RAID-4 systems, with the difference that the parity information is striped over all the disks with the data. For example, first, second, and third disks may each contain data and parity in interleaved fashion. Distributing the parity data generally increases the throughput of the array as compared to a RAID-4 system. RAID-5 systems may continue to operate though one of the disks has failed. RAID-6 systems are like RAID-5 systems, except that dual parity is kept to provide for normal operation if up to the failure of two drives.
Combinations of RAID systems are also possible. For example, a four disk RAID 1+0 system provides a concatenated file system that is also redundant. The first and second disks are mirrored, as are the third and fourth disks. The combination of the mirrored sets forms a storage space that is twice the size of one individual drive, assuming that all four are of equal size. Many other combinations of RAID systems are possible.
Software logic executing on a data storage system may send data input/output (I/O) requests to a RAID group. The software logic utilizes a device driver associated with the RAID group for sending the I/O requests, and receiving any corresponding responses. A device driver is a software or firmware that contains the information necessary for communicating with a specific hardware device or system. In commonly used data storage systems, the software logic sends a request to the device driver, and the device driver sends a corresponding request to the device or system. Any response from the device to the software logic may traverse the same path in reverse order.
Device drivers for data storage devices employ certain techniques for processing the I/O requests for the RAID group. For example, a data storage device driver may use a queue data structure to queue up the I/O requests received from elsewhere in the data storage system. The data storage device driver processes the I/O requests on a first in first out (FIFO) basis for the RAID group.